Matching circuit for megasonic transducer device

ABSTRACT

A method and apparatus for matching impedance magnitude and impedance phase for an acoustic-wave transducer load and an RF power source. The acoustic-wave transducer load has a load impedance magnitude and phase. The RF power source has a source impedance magnitude and phase. In one embodiment of the invention, a transformer matches the source and load impedance magnitudes. A capacitor, connected in series with the transformer, matches the source impedance phase to the load impedance phase.

This is a Divisional Application of Ser. No. 11/034,475 filed Jan. 12,2005 which is a Divisional Application of Ser. No. 10/194,174 filed Jul.12, 2002, now U.S. Pat. No. 6,954,021.

FIELD

The present invention relates generally to the field of semiconductortechnology and, more specifically, to matching circuits for megasonictransducers.

BACKGROUND

In semiconductor wafer substrate (wafer) cleaning, particle removal isessential. Particles can be removed by chemical means or by mechanicalmeans. In the current state of art, one means of removing particlesincludes the use of a megasonic cleaning device. A megasonic cleaningdevice utilizes a process wherein a wafer is placed in a liquid bath andhigh frequency (megasonic) irradiation, or cavitation, is applied to theliquid in the bath. At the same time, chemicals in the liquid provide aslight surface etching and provide the right surface termination, suchthat once particles are dislodged from the surface by the combination ofetch and mechanical action of the megasonics on the particles, theseparticles are not redeposited on the surface.

Various types of megasonic cleaning devices vary in diverse ways. Sometypes can clean one wafer at a time. Other types utilize very cleverways to reduce the amount of liquid used in the bath. Yet other typeswork at some frequencies better than others. Nonetheless, almost allmegasonic cleaning devices are similar in at least one way. They allmake use of an acoustic-wave transducer.

An acoustic-wave transducer is an electronic device that receiveshigh-frequency power signals (from an RF power source) that excite thetransducer and cause it to vibrate. The vibration causes sonic waves totravel through the liquid bath and provide the mechanical means toremove particles from the wafer surface.

For several reasons, however, power signals are not always successfullydelivered to the acoustic-wave transducer in the most efficient manner.To properly deliver power to an acoustic-wave transducer, the resistanceof the power source must match that of the transducer load. However, forthe transducer to function properly, the power signals must be of asinusoidal nature, such as alternating current (AC). As a consequence ofutilizing AC, reactive circuit elements, such as the transmission lineswithin the megasonic cleaning device, create impedance, and impedancematching is more difficult to accomplish than mere resistance matching.

Some approaches have nonetheless been attempted to match the impedanceof an acoustic-wave transducer load to the impedance of an RF powersource. One common approach has been to utilize an impedance-matchingtransformer. A transformer is an electronic device with two wires woundaround a magnetic core. The wires wound around the core are called“windings.” Typically, the coil connected to the AC power source iscalled the primary winding, while the coil connected to the load iscalled the secondary windings. The number of windings in the primarycoil compared to the number of windings in the secondary coil is calledthe “turns ratio.” A well known quality of the transformer is that theturns ratio squared is proportional to the impedances of circuitsconnected to the primary and secondary windings as follows:(N_(P)/N_(S))²; Z_(P)/Z_(S), where N_(P) is the number of primarywindings, N_(S) is the number of secondary windings, Z_(P) is theimpedance seen at the input of the transformer on the primary side andZ_(N) is the impedance seen at the input of the transformer on thesecondary side.

Thus, one approach for matching impedances of transducer loads to thatof an RF power source has been to connect the RF power source to theprimary windings of a transformer, connect the transducer load to thesecondary windings, then to manipulate the turns ratio until theimpedances are matched as seen from both sides. This approach, however,has serious detrimental side effects.

For example, the transformer, itself, introduces a good deal ofinductance into the circuit, thus increasing the reactance of thecircuit. As the reactance goes up, so does the impedance. Impedance ismeasured in two parts, however, magnitude and phase. Unfortunately,while the transformer works to correct impedance magnitude, it alsochanges phase. Impedance phase can also affect power loss between thepower source and the transducer load. In time, as electronic devices ona silicon wafer decrease in size, so do the particles that needcleaning. As a result, the transducer assembly must create cavitation athigher frequencies. A common way to create higher cavitation frequenciesis to use a larger transducer. The larger the transducer, however, thelower the impedance it provides, and, as a result, the higher the turnsratio must be in the transformer to match the power source impedance. Asthe turns ratio increases in the transformer, so does the inductance,and, consequently, so does the phase-difference in the power signals.The more the phase signals get out of phase, the less efficient is thepower transfer.

SUMMARY

A matching network circuit for a megasonic transducer is described. Inembodiments of the invention described herein, the matching circuitmatches both the impedance magnitudes and the impedance phases of an RFpower source and an acoustic-wave transducer load. By matching both theimpedance magnitude and the impedance phase, the acoustic-wavetransducer, on either a single or multiple wafer megasonic cleaningdevice, can operate at peak power efficiency during operation.Furthermore, the matching circuit can compensate for changes to thetransducer load, thus providing optimal power savings over a bandwidthof operating frequencies.

An exemplary matching circuit may include an impedance-phase-matchingcapacitor, such as a variable capacitor, to adjust phase-differences inAC power signals traveling between the RF power source and theacoustic-wave transducer load, and an impedance-magnitude-matchingtransformer, such as an autotransformer, to adjust the impedancemagnitude of the acoustic-wave transducer load to match the impedancemagnitude of the RF power source.

Other features of the present invention will be apparent from theaccompanying drawings and from the detailed description that follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and should not belimited by the figures of the accompanying drawings in which likereferences indicate similar elements and in which:

FIGS. 1A-1C illustrates exemplary one-wafer megasonic cleaning devicewith an exemplary acoustic-wave transducer assembly for cleaning onewafer at a time.

FIG. 2 is an exemplary megasonic cleaning device for cleaning multiplewafers.

FIG. 3 is an illustration of a circuit connecting an acoustic-wavetransducer load to an RF power source.

FIG. 4 illustrates one embodiment of a matching circuit according to thepresent invention.

FIG. 5 is a flow diagram of one embodiment of a method for matching bothimpedance magnitude and impedance phase for a megasonic acoustic-wavetransducer load.

FIG. 6 is an illustration of a resonance curve and phase-differencecurve for an acoustic-wave transducer load over a range of operatingfrequencies before applying the method described in FIG. 5.

FIG. 7 is an illustration of a resonance curve and a phase-differencecurve for an acoustic-wave transducer load over a range of operatingfrequencies after applying the method described in FIG. 5.

FIG. 8 shows a resonance curve and phase curve for an acoustic-wavetransducer load if the phase-adjusting variable capacitor were notincluded in the matching circuit.

FIG. 9 is a bar graph diagram demonstrating the efficiency of particleremoval from a 300 mm wafer, according to the present invention.

DETAILED DESCRIPTION

In the following description numerous specific details are set forth.One of ordinary skill in the art, however, will appreciate that thesespecific details are not necessary to practice embodiments of theinvention. While certain exemplary embodiments have been described andshown in the accompanying drawings, it is to be understood that suchembodiments are merely illustrative and not restrictive of the currentinvention, and that this invention is not restricted to the specificconstructions and arrangements shown and described since modificationsmay occur to those ordinarily skilled in the art. In other instanceswell-known acoustic energy cleaning and/or fabrication processes,techniques, materials, equipment, etc., have not been set forth inparticular detail in order to not unnecessarily obscure embodiments ofthe present invention.

Described herein is a matching circuit for a megasonic transducerutilized in conjunction with a megasonic cleaning device. The megasoniccleaning device includes an acoustic-wave transducer to clean particlesfrom a wafer. The acoustic-wave transducer is to deliver acoustic energyinto a cleaning fluid which causes cavitation (bubbling) to occur andgently brush away the particles from the wafer surface. Theacoustic-wave transducer receives power signals from an RF power source.The impedance of the acoustic-wave transducer (“load”) and the impedanceof the RF power source (“source”) may require matching. Consequently,the matching circuit, connected between the source and the load, is tomatch the impedance of the source to that of the load. In embodiments ofthe invention described herein, the matching circuit matches both theimpedance magnitudes and the impedance phases of the source and load. Bymatching both the impedance magnitude and the impedance phase, amegasonic cleaning device can operate at peak power efficiency duringmegasonic cleaning, even for a bandwidth of operating frequencies.According to one embodiment of the invention, the matching circuit mayinclude a phase-matching capacitor to adjust impedance phase matchingand a magnitude-matching transformer to adjust impedance magnitudematching.

In a megasonic cleaning device, an acoustic-wave transducer is utilizedto provide sound wave agitation to a liquid contained within a cleaningchamber. The liquid is typically in the form of a bath, but may also bein the form of a stream or a pulse of liquid. A silicon wafer rests on aplatter and is placed in the liquid bath, or in the path of the flowingliquid, and the transducers are activated. When the acoustic-wavetransducer is activated, power signals cause a piezoelectric material tobecome excited and produce sound waves at a certain frequency. When thesound waves encounter the cleaning liquid, they produce pressureoscillations that form bubbles that are continuously expanding andcollapsing as a result of the pressure oscillations. This expanding andcollapsing of bubbles is known as “cavitation”. Many of the bubblescollapse onto the surface of the silicon wafer and brush away looseparticles that may reside on the surfaces of the wafer.

The size of the bubbles depends on the frequency at which thepiezoelectric crystal vibrates. The lower the frequency, the lower arethe pressure oscillations in the liquid, thus allowing the bubbles to belarger. The effect of a large bubble collapsing will produce a moreforceful cleaning effect on the wafer surface. A drawback to lowfrequency cavitation, however, is that the cavitation is more random. Asthe frequency of the vibration of the transducer increases, however, thegreater are the pressure oscillations, and the bubbles are not allowedto get very large. The effect of a small bubble collapsing provides amuch more gentle cleansing, and, as a result, can be used for sensitivewafers. Also, if bubbles are smaller, then they can more efficientlyclean smaller devices and cavitation can be more controlled.

Currently, cavitation is divided into two viable categories: ultrasonicand megasonic. Ultrasonic frequencies range from approximately between20-400 kHz. Ultrasonic cleaning is useful for cleaning wafers with largedevices and that are less sensitive, since ultrasonic frequenciesproduce larger cavitation bubbles. Megasonic cleaning uses higherfrequencies, beginning at between 350-400 kHz and ranging well into theMHz frequencies. Megasonic frequencies do not cause the more forcefulcavitation effects found with ultrasonic frequencies. Megasonic cleaningsignificantly reduces, or eliminates, cavitation erosion and thelikelihood of surface damage to the wafer. Thus, in general, the higherthe frequency, the lower the damage to the wafer.

Also, since bubble size is less random for megasonic frequencies,megasonic cleaning produces more controlled cavitation. Controlledcavitation becomes acoustic streaming which can push the particles awayso they do not reattach to the wafer. Megasonic cleaning may be improvedby varying and/or pulsing the input power to the megasonic transducers,which can provide better control over cavitation than applying powercontinuously at a constant level. Megasonic cleaning may be improvedthrough the use of a plurality of frequencies to be simultaneouslygenerated, or by changing one or more frequencies during a cleaningcycle, during a rinsing cycles, or any combination thereof. Megasoniccleaning may also be improved through a selection of the frequency orfrequencies used. The control of power to the transducers, and through arange of megasonic frequencies, therefore, becomes an important aspectto the performance of a megasonic cleaning device.

FIG. 1A is an illustration of one embodiment of a megasonic cleaningdevice 100 that cleans one wafer at a time. The megasonic cleaningdevice is housed within a cleaning chamber 104 characterized by achamber housing 160. To initiate a wafer process cycle, a rotatablewafer holding bracket 148 translates along an axis 145 a distanceupward. A robot arm (not shown) holding a wafer 106 enters the interiorof the chamber 104 through an access door 158 and the wafer 106 isplaced in the bracket 148. The bracket 148 is then lowered so as toalign the wafer 106 horizontally a distance from a circular platter 108.The wafer 106, resting in the bracket 148, is parallel to the platter108 and located a distance from the platter 108, i.e., the gap. Theplatter 108 is flat where it faces the wafer 106 and, therefore, thedistance separating the platter 108 and wafer 106 is uniform.

Within the cleaning chamber 104, megasonic energy is generated by atleast one acoustic-wave transducer (“transducer”) 102 attached to theplatter 108 and the megasonic energy can pass into the wafer 106 throughchemicals 112 in contact with both the wafer 106 and the platter 108. Asa result, the wafer 106 can be cleaned with a variety of combinationsthat include wafer rotation, megasonic energy, and chemical action, allunder temperature control. Between and after cleaning and rinsingcycles, the megasonic cleaning device 100 can dry the wafer 106.

The platter 108 has a topside 117 and a bottom side 119, with the atleast one transducer 102 positioned in acoustic proximity to theplatter. In one embodiment of the invention, as shown in FIG. 1A, thetransducer 102 (or transducers) are attached to the bottom side 119 ofthe platter 108. The platter topside 117 can be facing the wafer 106. Inone embodiment, the platter 108 is fixed, but alternate embodiments canhave the platter 108 able to translate along the bracket rotation axis145 to open the gap during wafer rinse or dry cycles. The wafer 106 canbe placed in the rotatable wafer holding bracket 148 such that the waferdevice side 116 is facing up and away from the platter 108. The wafer106, when positioned in the bracket 148, can rest on three or morevertical support posts 110 of the bracket 148. When placed in thebracket 148, the wafer 106 can be centered over and held substantiallyparallel to the platter 108 to create the gap. The gap distance isapproximately 3 mm but can fall within the range of approximately 1-5mm. Gravity and the downward flow of air 123 from a filter 111 can actto maintain the wafer 106 positioned on the posts 110. Positionedbeneath the platter 108 can be an electric motor 122 for rotating thebracket 148. A through hole 185 can exist in the electric motor 122through which is passed the wiring 146 from the platter 108 as well as atube 128 that can transfer a first set of chemicals 112 (“firstchemicals”) to a feed port 142. The feed port 142 has an approximate0.190″ diameter. The feed port 142 can be located at the center of theplatter 108 or the feed port 142 can be placed off-center by up to a fewmillimeters (not shown).

Attached to the acoustic-wave transducer 102 can be a copper spring 144.The spring 144 could be of a variety of shapes to maintain electricalcontact such as a wire coiled shape (shown) or a flexed foil constructedfrom sheet metal (not shown). Soldered to the spring's 144 free ends arethe wiring's leads, thus forming electrical connections. The platter 108can be connected to the cleaning chamber 104 so as to act as ground forthe electrical connections to the acoustic-wave transducer 102 atsprings 144.

In one embodiment, located above the platter 108 and the wafer 106, maybe positioned a nozzle 151. Through the nozzle 151 can pass a second setof chemicals 126, 124, 125, and 127 (“second chemicals”) duringprocessing. The nozzle 151 can direct a fluid flow 150 onto the waferdevice side 116 with each of the chemicals 126, 124, 125, and 127 in thecleaning process. The nozzle 151 can apply the chemicals 126, 124, 125,and 127 to the wafer 106 while the wafer 106 is not moving or while thewafer 106 is spinning. The nozzle 151 can apply the chemicals 126, 124,125, and 127 at a flow rate to maintain a coating of the chemicals 126,124, 125, and 127 on the wafer device side 116 surface with minimalexcess.

The nozzle 151 can apply a continuous chemical flow to maintain a filmthickness on the wafer 106 of at least 100 microns. To keep the chemicalfilm at the 100 microns thickness, the chemicals 126, 124, 125, and 127may be converted at the nozzle 151 into a mist having a particular meandiameter droplet size. All nozzle designs are limited as to how small adroplet size they can create. To meet the requirements of minimal fluidusage, a further reduction in droplet size may be required. One methodof reducing the droplet size beyond a theoretical limit is to entrain,or dissolve, a gas, such as H₂ gas 105 or any other gas from the groupof O₂, N₂, Ar, or He into the chemicals 126, 124, 125, and 127.

Referring still to FIG. 1A, the bracket 148 and the wafer 106 arerotated while the first chemicals 112 is applied from below to be insimultaneous contact with the platter 108 and the non-device side of thewafer 114. The second chemicals 126, 124, 125, and 127 are wetted outonto the device side 116 of the wafer 106. The acoustic-wave transducer102 generates megasonic waves through the platter 108 into the firstchemicals 112, captured by the wafer 106 and the platter 108. Themegasonic waves may be incident to the wafer non-device side 114 at anangle substantially normal (perpendicular) to the wafer surface 114. Apercentage of the megasonic waves, depending on the frequency orfrequencies used, can pass through the wafer 106 to exit the waferdevice side 116 and enter the second chemicals 126, 124, 125, and 127that may be deposited as a film on the wafer device side 116. Themegasonic waves acting within the second chemicals 126, 124, 125, and127 can produce cleaning on the wafer device side 116. For optimalthroughput speed, the total area of the acoustic-wave transducer 102 canbe sufficient to provide approximately between 80-100% area coverage ofthe platter surface 119. The platter 108 diameter may be approximatelythe same size or larger than the wafer 106 diameter. The cleaning device100 is scalable to operate on a wafer 106 that is 100 mm (diameter), 300mm (diameter), or larger in size. If the wafer diameter is larger thanthe platter diameter, the vibrations from the megasonic energy strikingthe wafer 106 can still travel to the wafer 106 outer diameter (OD)providing full coverage for the cleaning action.

During the cleaning, rinse and dry cycles, the wafer 106 is rotated at aselected revolution per minute (rpm) about an axis 145 that runs throughthe bracket 148. Additionally, to optimize any particular cycle, thewafer spin rate may be stopped or varied and the sonic energy varied bychanging any combination of the power setting, the frequency orfrequencies, and by pulsing. Therefore, when the bracket 148 is inoperation, the wafer 106 is seeing the first chemicals 112 on thenon-device side 114, the second chemicals 126, 124, 125, 127 on thedevice side 116, while the wafer 106 is being rotated and radiated withmegasonic energy.

Acoustic waves can first strike the wafer non-device side 114 where nodevices 121 exist that could be damaged by the full force of theacoustic energy. Depending on the frequency or frequencies used, themegasonic energy may be dampened to a degree when passing through theplatter 108 and wafer 106 to exit into the cleaning or rinse chemicals126, 124, 125, and 127 at the wafer device side 116. As a result, themegasonic energy striking the wafer non-device side 114 may be powerfulenough that only de-ionized (DI) water is used as the first chemicals112.

A thin film (not shown) of the second chemicals 126, 124, 125, and 127may be applied to wet the wafer device side 116 surface. If not DI water125, the second chemicals 124, 126 and 127 may be a stronger chemistrysuch as used in an RCA (Radio Corporation of America) cleaning process.The action of the megasonic energy on the device structures 121 isconfined to a small volume (thin film) that contacts the devicestructures 121, absorbs the sonic waves, and maintains usefulcavitation.

In an embodiment, megasonic energy is applied to the rotating wafer 106throughout the cleaning process. The megasonic energy is in a frequencyrange of 400 kHz-8 Mz but may be higher.

A drain 162 may be provided within the cleaning chamber housing 160 tocollect the cleaning fluids. A cleaning chamber floor 163 may be angledtoward the drain 162 to improve flow of the chemicals 112, 126, 124,125, and 127 to the drain 162.

A matching circuit 170 connects the lead wires 146 to an RF power source174, via a transmission line 176. In one embodiment, a transducer, orplurality of transducers, may be used with the megasonic cleaning device100 that functions at only one frequency. Consequently, only onematching circuit 170 may be necessary. On the other hand, if more thanone type of transducer is utilized, additional matching circuits andpower sources may be necessary. The RF power source 174 delivers ACpower signals to the transducer 102. The transducer 102, in combinationwith other surrounding elements, has a load impedance magnitude andphase. The RF power source has a source impedance magnitude and phase.The matching circuit matches the load impedance magnitude and phase ofthe transducer assembly to the source impedance magnitude and phase.

FIG. 1B is an illustration of a cross-section of one embodiment of anacoustic-wave transducer assembly 200 utilized in conjunction withmegasonic cleaning device 100. The acoustic-wave transducer assembly 200includes at least one transducer 102, connected to the platter 108. Theplatter 108 can be made of aluminum that is polished and may have anapproximate diameter of 300 mm. Alternatively, it should be noted thatthe platter 108 could be made from a variety of materials such assapphire, stainless steel, tantalum, or titanium. The platter 108 isapproximately 3.43 mm thick (230) and the platter from side 217 can becoated with a protective fluoropolymer 234 such as Halar® ((AusimontUSA, Thorofare, N.J.), having a coating thickness (236) of between0.015-0.045″. The platter backside 214 can have one or moreacoustic-wave transducers 102 bonded directly to the aluminum platterwith an electrically conductive epoxy adhesive or a solder having anadhesive/solder thickness 240 of approximately 0.001-0.010″. Theopposite side of each of the one or more acoustic-wave transducers 102can be flexibly attached by springs 144 to electrical wiring 146 toprovide power at a frequency while the platter 108 can be connected toground. One ordinarily skilled in the art will recognize that transducerdesign is varied and that the transducer may be any other type known inthe art.

Transducer thickness t can be sized to generate sound at a particularfrequency. When a signal, generated at the frequency for which thetransducer has been designed to respond, arrives at the transducer, thetransducer will vibrate at that frequency. For example, a typicalacoustic-wave transducer is made form a piezoelectric material having athickness of 0.098″, which is designed to respond to a frequency of 920kHz. The piezoelectric material may be one of many well-known materialssuch as lead zirconate titanate (PZT), barium titanate or polyvinylidenefluoride resine (PVDF).

As previously explained, the effectiveness of cleaning by sound, inparticular removing particles, can be related to frequency, anddifferent sized particles can be more effectively removed with differentmegasonic frequencies. Currently, a large percentage of the particles tobe removed from a wafer exist in the 0.3 μm (micron) and 0.1 μm sizes.It has been determined that in cleaning wafers, the megasonic removal ofparticles in the 0.3 μm size range is efficient in the 925 kHz range,while the megasonic removal of particles in the 0.1 μm range isefficient in the 1.8 MHz range.

For a 300 mm wafer 106, the frequency of 5.4 MHz has a special utilityin that the 300 mm wafer 106 is transparent for those sound waves. At5.4 MHz±30%, the sound waves can travel substantially through the wafer106 to exit the opposite wafer surface. To obtain a frequency of 5.4MHz, the thickness of the acoustic-wave transducer 102, as well as eachthickness of all the other layers (platter 108 and adhesive/solder 240,FIG. 1B), are multiplied by a factor 920/5400=0.17 or alternatively thelayer thicknesses of the acoustic-wave transducer piezoelectricmaterial, adhesive, and aluminum platter are to be divided by a factorof 5.87. This will provide for a transducer to respond to a frequency of5.4 MHz and for a reduced bounce back from the other layers of materials108 and 240 that the sound must pass through on its way to the wafer106.

The use of two frequencies has been given in the above embodiments forpurposes of example, however, it should be appreciated that any numberof different frequencies could be provided and that the percent ofcoverage from each transducer type producing each of the frequenciescould be varied.

Thus, frequency range is important to effective particle removal on awafer. Consequently, a variety of transducers of different sizes andshapes can be attached to the platter 108, each responding to differentresonance frequencies. FIG. 1C illustrates just one configuration of avariety of possible transducer placement arrangements to transfermultiple frequency acoustic energy to the wafer. FIG. 1C illustrates theplatter 108 having two groups of transducer 250 and 252 in diagonalquadrants.

The embodiments shown in FIGS. 1A-1C are configured to clean one waferat a time. One ordinarily skilled in the art will recognize, however,that other well known types of megasonic cleaning devices exist that canclean multiple wafers at a time in a variety of different ways, and thatmay also be used in conjunction with any of the embodiments of thepresent invention. For example, FIG. 2 is an exemplary megasoniccleaning device 1000 for cleaning multiple wafers. It comprises acleaning chamber 1002 filled with a liquid bath 1004. Multiple wafers1006 are immersed in the bath liquid 1004. A transducer assembly 1008,including at least one megasonic acoustic-wave transducer, is connectedto an RF power source 1010, to receive AC power signals through thetransmission line 1012. The transducer assembly 1008 is in physicalproximity to the cleaning chamber 1002 so that, once activated, thetransducer assembly will transfer acoustic energy into the liquid bath1004, causing cavitation to clean particles from the surfaces of thewafers 1006. A holder 1014 maintains the wafers 1006 in position withinthe cleaning chamber 1002. As shown in FIG. 2, the wafers 1006 arepositioned so that the wafer surfaces 1016 are vertical (perpendicularto) to the side 1018 of the transducer assembly 1008 that faces towardsthe wafers 1006. However, one ordinarily skilled in the art willrecognize that the wafers 1006 may be positioned so that the surfaces1016 are horizontal (parallel to) to the face 1018 of the transducerassembly 1008, or at any other angle or position advantageous to wafercleaning. Furthermore, the transducer assembly 1008 may be positioned atdifferent angles, or on various sides, of the chamber 1002.

A matching circuit 1020 is connected between the transducer assembly1008 and the RF power source. The transducer assembly 1008 has a loadimpedance magnitude and load impedance phase. The RF power source 1010has a source impedance magnitude and source impedance phase. Thematching circuit 1020 matches the load impedance magnitude and loadimpedance phase of the transducer assembly 1008 to the source impedancemagnitude and source impedance phase of the RF power source 1010.

FIG. 3 is an illustration of circuit connecting an acoustic-wavetransducer load 302 to an RF power source 310. The term “load” isutilized herein to discuss the delivery of electronic power from a powersource to a circuit element, for example, any of the acoustic-wavetransducers and/or transducer assemblies of FIGS. 1A-1C and FIG. 2. Theacoustic-wave transducer load 302 is connected to a radio frequency(“RF”) generator 306 via a transmission line 308. The RF power generator306 is to deliver an AC power signal to the acoustic-wave transducerload 302 at a megasonic frequency. The RF generator 306 has an impedanceZ_(G). The transmission line 308 may also have an impedance Z_(LINE). Inone embodiment, the dimensions and constitution of the transmission line308 are designed to match the impedance of the power generator 306(i.e., Z_(G)=Z_(LINE)). In one embodiment, the transmission line is a 50ohm coaxial cable and the power generator 306 has as impedance magnitudeof 50 ohms. Herein, from an impedance viewpoint, the transmission line308 and RF power generator 306 should always be considered matched, and,consequently, the term “RF power source”, or “source”, may be utilizedin reference to the RF power generator 306 and the transmission line 308collectively. The RF power source 310 impedance is characterized asZ_(S) (“source impedance”) and equals to 50 ohms impedance magnitude inone embodiment of the invention. Typically, RF power generators andtransmission lines (e.g., coax cables) are designed with a fixedimpedance magnitude of 50 ohms.

The acoustic-wave transducer load 302 has impedance, Z_(L), which willherein be referred to as the “load impedance.” The source impedance,Z_(S), and load impedance, Z_(L), are expressions representing theopposition that the RF power source 310 and acoustic-wave transducerload 302 give to the AC power signal as the signal travels through thetransmission line 308. Acoustic-wave transducers are designed for a widevariety of applications utilizing megasonic frequencies and thereforethe acoustic-wave transducer may vary in shape, size, materials, etc.Consequently, depending on the physical characteristics of theacoustic-wave transducer and the application for which the acoustic-wavetransducer is used, the load impedance Z_(L) may vary. An advantage ofthe matching circuit 304, however, described in further detail inconjunction with exemplary matching circuit 400 in FIG. 4 below, is thatregardless of the actual acoustic-wave transducer used, the matchingcircuit 304 can be adjusted to suit the particular situation.

In the presence of an AC power signal, two parts characterize the sourceand load impedances: impedance magnitude and impedance phase. Impedancemagnitude is the result of resistive opposition that the AC power signalencounters as the signal courses through the atoms of the load 302,generator 306, line 308, and matching circuit 304. The impedance phase,on the other hand, is not a physical opposition caused by physicalresistance. Rather impedance phase is the result of the storage andrelease of energy in the load 302, generator 306, line 308, and matchingcircuit 304, as sinusoidal current and voltage waveforms course throughthem. However, because of the sinusoidal nature of the current andvoltage waveforms, inductance and/or capacitance of the load 302,generator 306, line 308, or matching circuit 304, will cause thesinusoidal current and voltage waveforms to be out of phase with eachother.

Both impedance magnitude and impedance phase can contribute tonon-optimal power transfer of the power signals between the load 302 andthe source 310. When the impedance magnitude of the load 302 and source310 are different, or when the current and voltage waveforms are out ofphase, then power transfer between source 310 and load 302 isnon-optimal.

For this reason, the matching circuit 304 is introduced between thesource 310 and the load 302. The matching circuit 304 is to match thesource and load impedance magnitudes for a power signal transmitted fromthe RF power source 310 at a given megasonic frequency. Furthermore, thematching circuit 304 is to match the source and load impedance phases byadjusting the current and voltage waveforms to the same phase, so thatimpedance phase and impedance magnitude are matched at the samemegasonic frequency.

FIG. 4 illustrates an exemplary matching circuit 400 according to thepresent invention. Referring to FIG. 4, the matching circuit 400includes an impedance-phase-adjusting capacitor 402, which may also bereferred to as an “impedance-phase-matching capacitor”, a“phase-matching capacitor” or any other term indicating an ability tomatch impedance phase with a capacitor. The impedance-phase-adjustingcapacitor is connected in series with an impedance-magnitude-adjustingtransformer 404, which may also be referred to as an“impedance-magnitude-matching transformer”, an “impedance transformer”,or any other term indicating an ability to match impedance magnitudewith a transformer. The impedance-magnitude-adjusting transformer 404 isto match the load impedance magnitude to the source impedance magnitude,and the impedance-phase-adjusting capacitor 402 is to match the loadimpedance phase to the source impedance phase.

As shown in FIG. 4, the impedance-magnitude-adjusting transformer 404may be an autotransformer 404, which varies from a more common type oftransformer known as a power transformer in that a power transformer hastwo distinct primary and secondary windings. An autotransformer, on theother hand, has a single coil that is “tapped” to produce what iselectrically a primary and secondary winding. One ordinarily skilled inthe art will recognize, however, that other kinds of transformers, orother electronic devices, may be utilized to provide impedance magnitudematching. Referring to FIG. 4, the coil 406 has a primary windingportion 408 between a top node 410 and bottom node 412. The bottom nodemay be connected to ground. The coil 406 also has a secondary windingportion 414 between a tap node 416 and the bottom node 412. In oneembodiment of the invention, the top node 410 and bottom node 412 areconnected to the RF power source, while the tap node 416 and the bottomnode 412 are connected to the acoustic-wave transducer load. The primarywinding portion 408 between the top node 410 and the bottom node 412contains a first number of windings (N₁) to provide a primary electricalwinding, or primary winding. The secondary winding portion 414 betweenthe tap node 416 and the bottom node 412 contains a second number ofwindings (N₂), less than the first number of windings (N₁), to provide asecondary electrical winding, or secondary winding. The comparison ofthe secondary winding to the primary winding is the turns ratio(n=N₁/N₂). By adjusting the turns ratio, the impedance magnitude of thepower source and the load can be matched.

Depending on the application for which the matching circuit 400 is used,the turns ratio for the autotransformer 404 can be adjusted. Impedancemagnitude matching can be determined depending on the particularapplication for which the matching circuit 400 is used. Most often theimpedance magnitude of the load will be predetermined since theapplication for which the acoustic-wave transducer will be known and theacoustic-wave transducer will not generally vary. Consequently, in oneembodiment of the invention, the turns ratio can also be predeterminedbefore operation of the circuit is commenced and the tap node 416, andhence the turns ratio, may be fixed, hardwired, etc., to a particularposition for any given matching circuit used for a particularapplication. However, in other embodiments of the invention, where theimpedance of the load is varied (e.g., if the transducer varies), thenit may be advantageous to keep the tap node 416 unfixed so that it mayadjusted during application set-up or during operation.

In one embodiment of the invention, as shown in FIG. 4, theimpedance-phase-adjusting capacitor 402 is a variable capacitor 402connected in series with the autotransformer 404 at the primary winding408. One ordinarily skilled in the art, however, will recognize thatother types of capacitors, other than variable capacitors, may be used.The value of the variable capacitor 402 can be adjusted to increasecapacitance, which counteracts inductance introduced by theautotransformer 404, the RF power source, and the acoustic-wavetransducer load. The higher the value the variable capacitor is tunedto, the more inductance it can counteract until the phase-difference ofcurrent and voltage waveforms, is zero, or as close to zero as possible.If not for the variable capacitor 402, the inductance would overwhelmthe circuit and cause undesirable phase-differences in the power signalsvoltage and current.

The variable capacitor 402 is useful to adjust phase-differences forprocesses and assemblies having a variety of different load impedancesand/or frequencies. As described above, acoustic-wave transducers aredesigned for a wide variety of applications utilizing megasonicfrequencies and therefore the acoustic-wave transducer may vary inshape, size, materials, etc. Consequently, depending on the physicalcharacteristics of the acoustic-wave transducer and the application forwhich the acoustic-wave transducer is used, the load impedance may vary.An advantage of the matching circuit 400 is that regardless of theactual acoustic-wave transducer used, the matching circuit 400 can beadjusted by varying the variable capacitor 402 to suit the particularsituation.

For example, a first transducer may be swapped out of a megasoniccleaning device and replaced with a second transducer. The impedancephase of the second transducer may vary from that of the firsttransducer. It would therefore be advantageous to not have the variablecapacitor 402 fixed so that the matching circuit 400 would not need tobe swapped, rather merely adjusted for new phase matching. In anotherexample, a transducer may simply experience wear and tear, causingphysical characteristics of the transducer to change, in effect causingthe impedance phase to also change. As a result, it may be advantageousto adjust the variable capacitor 402 to compensate. Furthermore, it mayeven be advantageous to adjust the variable capacitor 402 duringoperation of the transducer to optimize the cleaning process as itoccurs.

In many cases, however, the transducer is predetermined and the loadwill not change. As a result, one setting on the variable capacitor 402may be sufficient. Consequently, in one embodiment of the invention, thevariable capacitor 402 may be fixed, hardwired, etc., before operationof the transducer load, or the variable capacitor 402 may instead be anon-variable capacitor designed to a particular value necessary toprovide phase matching.

Furthermore, in the embodiment shown in FIG. 4, the variable capacitor402 is shown connected in series between the autotransformer 404 and theRF power source. However, one ordinarily skilled in the art willrecognize that the variable capacitor 402 could also be connected inseries between the autotransformer 404 and the load.

FIG. 5 is a flow diagram of one embodiment of a method 500 for matchingboth impedance magnitude and impedance phase for a megasonicacoustic-wave transducer load. The acoustic-wave transducer load has anassociated impedance resonance curve and an associated phase-differencecurve. The acoustic-wave transducer load is connected to an RF powersource via a matching circuit that includes animpedance-magnitude-adjusting transformer and animpedance-phase-adjusting capacitor. The method includes, as shown inprocessing block 502, selecting a turns ratio for theimpedance-magnitude-adjusting transformer, such as an autotransformer,to match the impedance of the RF power source to that of theacoustic-wave transducer load at a desired operating point between ananti-resonance peak and a resonance peak of the transducer's impedanceresonance curve. This may include adjusting the turns ratio to produce apeak in the resonance curve with an impedance magnitude greater than theimpedance magnitude of the power source. Method 500 further includes, asshown at processing block 504, selecting a capacitance value of theimpedance-phase-adjusting capacitor, such as a variable capacitor, sothat a phase-difference curve is positioned to approximate zerophase-difference around a bandwidth of frequencies near the desiredoperating point. Finally, method 500 may conclude, as shown atprocessing block 506, with transmitting AC power signals atapproximately the operating point frequency from the RF power source,through the matching circuit, to the acoustic-wave transducer load.Method 500 will be described in more detail in conjunction with FIGS.6-9 below.

FIG. 6 is an illustration of a resonance curve 602 and aphase-difference curve 604 for an acoustic-wave transducer load over arange of operating frequencies (e.g., 875 kHz-975 kHz), before applyingthe method 500. The acoustic-wave transducer load may include atransducer designed to operate at a particular resonance frequency, suchas 925 kHz. As such, the transducer's resonance curve may demonstrate ananti-resonance peak 606 at 925 kHz. However, the transducer may beoperated at any point along the resonance curve, for example, at thedesired operating point 610 corresponding to the frequency of 917 kHz.Theoretical and experimental simulations have shown that maximumtransducer efficiency, measured in terms of acoustic pressure, issomewhere in the middle of the resonance curve between resonance 608 andanti-resonance 606. However, the value of |Z| in the middle of theresonance curve is small, only about 4-5 ohms. If the desired point werechosen to be near the middle of the resonance curve a matching circuitwould require a transformer with a turns ratio so high that it wouldcause enormous losses due to heating caused by very high current flowthrough the secondary winding. For this reason, the desired operatingpoint is chosen to be in a range slightly above the middle of theresonance curve, but slightly below the anti-resonance peak 606. In theembodiment shown in FIG. 6, this range is anywhere within 5-8 ohms, andthe exact desired operating point is at 8 ohms. At that point, thecorresponding frequency of the desired operating point is 917 kHz. Inthe following embodiments, therefore, the term “desired operating point”will be utilized to indicate the point on the resonance curvecorresponding to the frequency of 917 kHz.

In one embodiment, the acoustic-wave transducer load may be used inconjunction with an RF power source, including a 50 ohm RF powergenerator and 50 ohm matched transmission line. The impedance magnitudedifference between the load (8 ohms) and the source (50 ohms) would be42 ohms—an enormous impedance magnitude difference. In addition, at thedesired operating point, the phase curve shows a phase-difference ofalmost 40 degrees (408). The circuit would encounter tremendous powerloss. Both impedance magnitude matching and impedance phase matching areneeded.

FIG. 7 is an illustration of a resonance curve 702 and aphase-difference curve 704 for an acoustic-wave transducer load over arange of operating frequencies, after applying the method 500. Amatching circuit is applied comprising an impedance-magnitude adjustingtransformer and an impedance-phase-adjusting capacitor. A turns ratio isselected for the transformer (as described in FIG. 5, processing block502) so that the impedance resonance curve of the acoustic-wavetransducer circuit produces a steep, nearly vertical, slope atapproximately the impedance of the RF power source (i.e., 50 ohms). Thismay require selecting the turns ratio so that a maximum peak, oranti-resonance peak 706, in the resonance curve is greater than theimpedance of the power source. In the embodiment shown in FIG. 7, theturns ratio is selected to produce a maximum peak approximately twicethe impedance of the power source, approximately 100 ohms. Theanti-resonance peak 706 should be high so that the slope of theresonance curve is steep near the frequencies where the impedancemagnitude of the RF power source matches that of the acoustic-wavetransducer load (i.e., around 50 ohms). At the same time, the minimumpeak, or resonance peak 708, should be well below where the impedancemagnitude of the RF power source matches that of the acoustic-wavetransducer load. The middle of the curve therefore approximates theimpedance of the RF power source around a range of frequencies. Thisfrequency range may be termed a “bandwidth”. The bandwidth existsbetween the resonance peak 708 and the anti-resonance peak 706,approximately where the impedance magnitude of the RF power sourcematches that of the acoustic-wave transducer load (i.e., around 50ohms). Depending on the transducer manufacturer, and the temperature andconstitution of the cleaning liquid, the bandwidth may be produced towithin a 4-5 kHz range in width, centered on the desired operatingpoint. The desired operating frequency still corresponds to 917 kHz, andthe desired operating point would be the point on the resonance curvecorresponding to 917 kHz. In the embodiment shown in FIG. 7, that on theresonance curve corresponds to an impedance magnitude of 54.6 ohms.Those ordinarily skilled in the art, however, will recognize that thetransformer's turns ratio can be adjusted to work at other desiredoperating points.

In practice, the turns ratio of the impedance-magnitude-adjustingtransformer may be selected before the actual operation of thetransducer if the load impedance of the transducer is known. Forexample, the methodologies described herein may be utilized duringsimulation, set-up, or calibration of a megasonic cleaning device. Theoperating point, therefore, is predetermined based on the kind ofmegasonic operation that will be performed and the type of transducerused during that operation. Consequently, the turns ratio may beselected and then fixed for future operation. Fixing the turns ratiodepends on when the method is performed. For example, during asimulation, the impedance-magnitude-adjusting transformer would be onlya software object in the simulation program and the turns ratio valuewould be variable in a computer program. During setup of the cleaningdevice, however, selecting the turns ratio may be done manually andtherefore may include moving the tap node for an autotransformer. Thetap node can be fixed by soldering it into the appropriate positioncorresponding to the selected turns ratio. The turns ratio may beadjusted during, or between, operations if the transducer load changessignificantly (e.g., by swapping out different sized transducers) andtherefore it may include unsoldering the tap node and moving it to adifferent position.

The capacitance value of the impedance-phase-adjusting capacitor isselected (as described in FIG. 5, processing block 504) to a value(e.g., 3 nanofarads to 5 nanofarads), so that the phase curveapproximates zero phase-difference around the bandwidth of frequencies.In one embodiment of the invention, this is accomplished by positioninga peak 710, or a point near the peak, of the phase-difference curveslightly above 0° phase-difference (e.g., around 58 phase-difference to100 phase-difference) at the same frequency that the resonance curveapproximately matches the impedance of the transmission line. In FIG. 7,the phase peak was placed at approximately 58 phase-difference, shown as5.38 phase-difference in FIG. 7, at a frequency approximating thedesired operating point and within the bandwidth. During operation ofthe transducer, the operating frequency may tend to shift away from thedesired operating point. Since the shape of the phase curve anglesdownward toward 0 8 phase-difference, and slopes gradually from about 58phase-difference to about −58 phase-difference over the bandwidth (e.g.,approximately 917 kHz 6 2.5 kHz), then the transducer experiences onlyabout 658 phase-difference over the bandwidth and is able to function atnear-optimal power transfer between source and load.

In practice, the load impedance magnitude and phases can bepredetermined, hence the capacitance value of theimpedance-phase-adjusting capacitor may be selected previous to theactual operation of the megasonic cleaning device, for example duringsimulation or actual set-ups and calibration of a megasonic cleaningdevice. The capacitance value of the impedance-phase-adjusting capacitormay be selected to provide the necessary capacitance necessary toposition the phase curve, and then the capacitance value may be heldfixed (e.g., hardwired) for the actual operation of the megasoniccleaning device. The benefit of the bandwidth is obvious whenconsidering that for a fixed capacitance value, actual operation of themegasonic cleaning device can operate at near optimal power efficiencyover the bandwidth of frequencies.

For example, in the process of cleaning silicon wafers (i.e., duringoperation), the acoustic-wave transducer may agitate liquid inside acleaning chamber; however, the liquid level may not remain steady duringoperation since waves in the liquid, produced by the transducer, mayfurther introduce slight changes to the impedance of the transducerload. Nevertheless, as shown in the FIG. 7, even if the load impedancewere to change slightly the resonance curve will still match tophase-differences very near to 0° phase-difference on the phase curve(within about +5° phase difference).

In addition, after the capacitance value of the variable capacitor isselected, the bandwidth that the megasonic cleaning device experiencesduring operation allows the RF power source to slightly shiftfrequencies and still encounter near-optimal power transfer. Forexample, the frequency of the power source may need to adjust toconditions within the cleaning chamber of a megasonic cleaning device,such as to a shifting load impedance value or to the rotation of thewafer on the platter. As a consequence, if the frequency were to changeslightly, then the phase-difference would still be very near to zero.Consequently, the matching circuit can provide near optimal powertransfer within the bandwidth during the operation of the cleaningdevice, resulting in a more robust power-saving cleaning device.

As mentioned above, the capacitance value of theimpedance-phase-adjusting capacitor could be selected before operationand held fixed during operation. It should be noted, however, that theimpedance-phase-adjusting capacitor may be adjusted during operation, orbetween operations, of the megasonic cleaning device if, for whateverreason, the impedance load were to change (e.g., by swapping outdifferent sized transducers) so that the operational efficiency was nolonger possible over the bandwidth, or to change the phase curve tomatch a different desired operating frequency. In an embodiment wherethe impedance-phase-adjusting capacitor is a variable capacitor, thenthe capacitance value of the variable capacitor may be adjusted, whennecessary, to shift the phase curve up or down, and therefore thebandwidth can be easily shifted simply by adjusting the variablecapacitor to higher or lower capacitance values. As the capacitancevalue increases, the phase curve shifts upward. As the capacitance valuedecreases, the phase curve shifts downward.

FIG. 8 is an illustration of a resonance curve 802 and phase curve 804for an acoustic-wave transducer load if an impedance-phase-adjustingcapacitor were not included in the matching circuit. Problems can beseen in the shapes of the resonance curve 802 and phase curve 804. Forexample, because of the large amounts of inductance introduced by thetransformer and the transmission line, the resonance curve is flipped sothat the anti-resonance peak 806 is at the bottom of the curve and theresonance peak 808 is at the top. Thus, the slope of the resonance curvebetween resonance 808 and anti-resonance 806 slopes away from thecorresponding frequency of the desired operating point (i.e. 917 kHz).The transducer would experience an optimal operating point around 922kHz, but it would be far from the desired operating point. On the otherhand, embodiments of the matching circuit described herein introduce animpedance-phase-adjusting capacitor, which has sufficient capacitance tocounteract the inductance of the transformer and the transmission line,thus returning the slope of the resonance curve to its originalorientation, and allows the transducer to operate near the desiredoperating point.

Another problem readily apparent in FIG. 8 is that the large amount ofinductance from the transformer inverts the shape of the phase curve 804and shifts the peak 810 away from the desired operating point, thusresulting in a phase curve with a steep slope around the optimalfrequency point, 922 kHz. Thus, if the frequency were to shift evenslightly away from 922 kHz, a very large phase-difference would result.For example, if the frequency were to shift slightly, to about 920 kHz,the phase-difference would balloon from 5.78 phase-difference to about408 phase-difference. If the frequency were to shift even a little more,for example to the frequency of the originally desired operating point,917 kHz, the phase-difference would become almost 608. Without thevariable capacitor, there is no way to correct the phase curve 804.Consequently, the phase could be grossly mismatched, thus resulting insignificant power transfer loss. Thus, embodiments of the presentinvention are advantageous because they allow a means to adjust thephase curve 804, resulting in a transducer that can operate at severaloptimal points along the transducer resonance curve, between theanti-resonance point and resonance point.

Another problem with the method of matching using only a transformer, asshown in FIG. 8, is that the method tries to match the maximum resonancepeak 808 to the exact RF power source impedance. To match impedanceexactly, however, requires very complex adjustments to be made to thetransformer, and allows for power transfer efficiency at only oneoptimal frequency point. On the other hand, via embodiments of thepresent invention, the process is made much easier, and more robust, byutilizing an impedance-phase-adjusting capacitor to adjust the phasepeak to provide nearly zero phase-shift along several near-optimalfrequency points that are between peaks. Plus, as stated further above,actual particle removal simulations indicate that operating the matchingcircuit on the slope between resonance and anti-resonance peaks, not atone of the peaks, leads to more optimal cleaning.

FIG. 9 is a bar graph diagram demonstrating the efficiency of particleremoval from a 300 mm wafer, according to the present invention.Referring to FIG. 9, percentages are shown comparing full and spotparticle removal between a megasonic wafer cleaner 902 utilizing amatching circuit with an impedance-phase-matching capacitor and amegasonic wafer cleaner 904 utilizing a matching circuit without animpedance-phase-matching capacitor. As shown in FIG. 9, for powertransmissions of 1000 W, the megasonic wafer cleaner utilizing amatching circuit with an impedance-magnitude-matching transformer and animpedance-phase-matching capacitor cleans a wafer significantly betterthan a megasonic wafer cleaner utilizing a matching circuit without animpedance-phase-matching capacitor. According to the graph, phasematching can improve efficiency up to 3% for full particle removal andup to 8% for spot removal of particles.

Several embodiments of the invention have thus been described. However,those ordinarily skilled in the art will recognize that the invention isnot limited to the embodiments described, but can be practiced withmodification and alteration within the spirit and scope of the appendedclaims that follow.

1. An apparatus for processing a wafer, comprising: a bracket forpositioning and rotating the wafer about an axis; a platter alignedbeneath and parallel to the bracket, with the platter having a throughhole, and the wafer having a first side facing the platter and a secondside facing away from the platter; a first fluid source connected to thethrough hole for flowing a first chemical within a gap between the firstside of the wafer and the platter; a second fluid source for flowing asecond chemical onto the second side of the wafer; at least oneacoustic-wave transducer, positioned on the platter, capable ofconverting AC power signals into acoustic energy to be transmitted intothe first and second chemicals, the at least one acoustic-wavetransducer having a load impedance magnitude and a load impedance phase;an RF power source, including an RF generator to produce the AC signalsand a transmission line to transmit them to the at least oneacoustic-wave transducer, the RF power source having a source impedancemagnitude and a source impedance phase; and a matching circuit connectedto the RF power source and the at least one acoustic-wave transducer, tomatch the load impedance magnitude and load impedance phase to thesource impedance magnitude and the source impedance phase.
 2. Theapparatus of claim 1, wherein the matching circuit includes a variablecapacitor connected in series with an autotransformer, theautotransformer to match the load impedance magnitude to the sourceimpedance magnitude and the variable capacitor to match the loadimpedance phase to the source impedance phase.
 3. The apparatus of claim2, wherein the variable capacitor is to provide capacitance tocounteract inductance introduced by the autotransformer and thetransmission line.
 4. The apparatus of claim 2, wherein the variablecapacitor is connected in series between either the autotransformer andthe RF power source or between the autotransformer and the at least oneacoustic-wave transducer.
 5. The apparatus of claim 2, wherein thevariable capacitor is to adjust a phase-difference curve to providenear-optimal power transfer within a bandwidth of 5 kHz.
 6. Theapparatus of claim 1, wherein the load impedance magnitude and phasefurther include impedance introduced by any one of the platter, thebracket, the first chemical, the second chemical, and the wafer.
 7. Theapparatus of claim 1, wherein the AC power signals are at a megasonicfrequency.
 8. The apparatus of claim 1, wherein the AC power signals arebetween approximately 875 to 975 kHz.
 9. A method, comprising: selectinga turns ratio for an impedance-magnitude-adjusting transformer toproduce a maximum peak in a resonance curve for an acoustic-wavetransducer load, the maximum peak being greater than the impedance of anRF power source so that a desired operating point on the resonance curveexists between the maximum peak and a minimum peak of the resonancecurve, the desired operating point approximating the impedance of the RFpower source around a bandwidth of frequencies; and selecting acapacitance value for an impedance-phase-adjusting capacitor to positiona phase-difference curve for the acoustic-wave transducer load toapproximate zero phase-difference around the bandwidth of frequencies.10. The method of claim 9, wherein the bandwidth of frequencies has arange of 5 kHz.
 11. The method of claim 9, wherein the turns ratio ofthe impedance-magnitude-adjusting transformer is selected to produce apeak in the resonance curve to approximately twice the impedancemagnitude of the RF power source.
 12. The method of claim 9, wherein theturns ratio of the impedance-magnitude-adjusting transformer is selectedto produce a slope of the resonance curve that is nearly vertical overthe bandwidth of frequencies.
 13. The method of claim 9, wherein thecapacitance value of the impedance-phase-adjusting capacitor is selectedso that a peak of the phase-difference curve is at approximately 58 to10° phase-difference at the frequency of the desired operating point.14. The method of claim 9, wherein the capacitance value of theimpedance-phase-adjusting capacitor is selected prior to operation ofthe acoustic-wave transducer load and the capacitance value is heldfixed during operation.
 15. The method of claim 9, wherein thecapacitance value of the impedance-phase-adjusting capacitor is adjustedduring operation or between operations.
 16. The method of claim 9,wherein the turns ratio of the impedance-magnitude-adjusting transformeris selected prior to operation of the acoustic-wave transducer load andheld fixed during operation.
 17. The method of claim 9, wherein theturns ratio of the impedance-magnitude-adjusting transformer is adjustedduring operation or between operations.
 18. A method, comprising:selecting a turns ratio for an autotransformer to match an impedancemagnitude of an RF power source to that of an acoustic-wave transducerload at a desired operating point between an anti-resonance peak and aresonance peak of an impedance resonance curve for the acoustic-wavetransducer load; and selecting a capacitance value for a variablecapacitor so that a peak of a phase-difference curve for theacoustic-wave transducer load is positioned slightly above 0°phase-difference near the desired operating point; and transmitting ACpower signals from the RF power source, through the autotransformer andthe variable capacitor, to the acoustic-wave transducer load.
 19. Themethod of claim 18, wherein the turns ratio is selected to produce theanti-resonance peak in the resonance curve with an impedance magnitudegreater than the impedance magnitude of the power source.
 20. The methodof claim 18, wherein the turns ratio of the autotransformer is selectedto produce a peak in the resonance curve to approximately twice theimpedance magnitude of the RF power source.
 21. The method of claim 18,wherein the variable capacitor is to adjust a phase-difference curve toprovide near-optimal power transfer within a bandwidth of frequencies.22. The method of claim 21, wherein the bandwidth is approximately 5 kHzin width.
 23. A method, comprising: placing a wafer device side up in abracket; positioning the bracket such that the wafer is substantiallyparallel to and separated from a platter; flowing a first chemicalbetween a first side of the wafer and the platter; rotating the wafer;applying a second chemical to a second side of the wafer; transmittingpower signals from an RF power source through a matching circuit to anacoustic-wave transducer attached to the platter, the transducer toproduce acoustic energy to be transmitted into the first chemical andthe second chemical, the RF power source having a source impedancemagnitude and phase and the acoustic-wave transducer having a loadimpedance magnitude and phase; and adjusting the matching circuit sothat the source impedance magnitude and phase approximately equal to theload impedance magnitude and phase.
 24. The method of claim 23, whereinthe matching circuit includes an autotransformer and a variablecapacitor, and the acoustic-wave transducer has an impedance magnituderesonance curve and a phase-difference curve, the method furthercomprising: selecting a turns ratio for the autotransformer to match theimpedance of the RF power source to that of the acoustic-wave transducerload at a desired operating point between an anti-resonance peak and aresonance peak of the impedance resonance curve.
 25. The method of claim23, further comprising: selecting a capacitance value for the variablecapacitor to position a peak of the phase-difference curve near thedesired operating point on the resonance curve.
 26. The method of claim23, wherein the turns ratio of the autotransformer is selected toproduce a peak in the resonance curve of approximately twice theimpedance of the power source.
 27. The method of claim 23, wherein theturns ratio of the autotransformer is selected to produce a slope of theresonance curve nearly vertical over a bandwidth of frequencies.
 28. Themethod of claim 27, wherein the bandwidth of frequencies has a range of5 kHz.